1. Field of the Invention
The present invention relates a semiconductor device suitably used for a photoelectric transducer such as a photocoupler or the like, a solid-state imaging device or field-effect imaging device comprising a semiconductor image sensor which receives light incident on an on-chip lens formed on a color filter, a method of manufacturing the semiconductor device, and an apparatus for manufacturing a semiconductor.
More specifically, a refractive index matching film is provided on a photoelectric conversion light-receiving element, and a composition composed of silicon, oxygen and nitrogen which constitute the refractive index matching film is adjusted so that the refractive index of a compound layer constituting the refractive index matching film continuously changes from the refractive index of a silicon oxide film of 1.45 to the refractive index of a silicon nitride film of 2.0. As a result, reflection from the light receiving element can be minimized, and light receiving sensitivity can be improved.
2. Description of the Related Art
In recent years, a video camera and a digital still camera have been increasingly used in many schools, homes and broadcast stations. Such a camera requires a solid-state imaging device. The solid-state imaging device comprises CCD (Charge Coupled Device) imaging devices arranged as photoelectric transducers in a two-dimensional form. The CCD imaging device means a semiconductor device having a structure in which unit elements each comprising a photodiode and a MOS capacitor are regularly arranged. The solid-state imaging device has the function to move a group of charges stored in the surface of a semiconductor substrate along the array direction of electrodes of the MOS capacitors.
Namely, the solid-state imaging device comprises pluralities of photodiodes, MOS capacitors, vertical transfer registers, horizontal transfer registers, and charge detecting amplifiers, which are provided on the semiconductor substrate. When light is applied to a light receiving surface of the solid-state imaging device, the light is converted into signal charges by the photodiodes, and then stored in the MOS capacitors. The signal charges stored in the MOS capacitors are transferred by the vertical transfer registers (referred to as “vertical CCD sections” hereinafter) and horizontal transfer registers, and finally detected by the charge detecting amplifiers and read as analogue received signals.
FIG. 14 is a sectional view showing an example of a configuration of a solid-state imaging device 10 of a first conventional example. As shown in FIG. 14, a semiconductor buried layer (P-WELL) 1 is formed on a N-type silicon substrate 11, the P-WELL 1 comprising photodiodes PD each having a N-type impurity region (impurity diffused layer) 2, and vertical CCD sections 12 each having a N-type impurity region (impurity diffused layer) 3. The P-WELL 1 further comprises transfer gate sections 13 for reading out signal charges from the photodiodes PD to the vertical CCD sections 12, to isolate the silicon substrate 11.
The N-type impurity region 2 constituting each of the photodiodes PD is isolated from the N-type impurity region 3 constituting the corresponding vertical CCD section 12 by a channel stopper 4 comprising a P-type impurity region. Furthermore, a transfer electrode 17 is provided on each of the vertical CCD sections 12 through a gate insulating film (silicon oxide film) 14.
The transfer electrodes 17 of the vertical CCD sections 12 are covered with a shielding film 19 composed of aluminum or tungsten through an interlayer insulating film 18. The shielding film 19 has apertures formed above the photodiodes PD to define light-receiving windows 21. The shielding film 19 is coated with a cover film 22 comprising a silicon oxide film of PSG or the like. Furthermore, a planarizing film 23, a color filter 24, and microlenses 25 are formed in order on the cover film 22.
The material of the cover film 22 is not limited to the silicon oxide film, and an example using a silicon nitride film is also known. For example, the technical document, Japanese Unexamined Patent Application Publication No. 60-177778, discloses that a plasma silicon nitride film is formed on a transparent electrode composed of polycrystalline silicon. However, in such a structure in which a silicon nitride film is deposited, an increase in short-wavelength sensitivity is expected due to a multiple interference effect.
Therefore, in the structure shown in FIG. 14 in which the silicon interfaces of the photodiodes PD are covered directly with the cover film 22, a loss of incident light is increased due to surface reflection from the N-type silicon substrate 11 to fail to obtain sufficient light receiving sensitivity.
In addition, in the structure in which the plasma silicon nitride film is formed below the planarizing film 23, ripple occurs in spectral transmittance due to an interference effect between a silicon nitride film serving as the interlayer insulating film 18 and a silicon nitride film serving as the gate insulating film 14 provided below the interlayer insulating film 18. Therefore, the spectral characteristics of the color filter layer 24 easily vary.
In order to solve the above-described problem, for example, Patent Publication No. 3196727 discloses a technique for forming an anti-reflection film on photodiodes PD. FIG. 15 is a sectional view showing an example of a construction of a solid-state imaging device 10′ of a second conventional example.
The solid-state imaging device 10′ shown in FIG. 15 comprises a N-type silicon substrate 11 on a surface of which photodiodes PD are formed for obtaining signal charges. Each of the photodiodes PD comprises a N-type impurity region (impurity diffused region) 2.
Furthermore, a silicon oxide thin film serving as a gate insulating film 14 is formed on the silicon substrate 11, and a silicon nitride film serving as an anti-reflection thin film 15 having a refractive index higher than that of the silicon oxide film 14 and lower than that of the silicon substrate 11 is formed on the silicon oxide thin film 14. The refractive index of the silicon oxide film 14 is about 1.45, and the refractive index of the silicon nitride film is about 2.0. Assuming that the refractive index is n, the thickness t of each of the silicon oxide film and the silicon nitride film is set to satisfy the relationship 350/(4n) nm≦t≦450/(4n) nm. These films 14 and 15 are formed for preventing a dark current.
When the thickness of each of the silicon oxide film and the silicon nitride film is set as described above, the anti-reflection film 15 having relatively flat reflection in the visible light region can be obtained. By appropriately setting the thickness of each of the silicon oxide film and the silicon nitride film, reflectance can be suppressed to an average of about 12 to 13%, and is thus suppressed to about ⅓ of the reflectance of the conventional silicon substrate 11 of about 40%.
Like in the first conventional example, transfer electrodes 17 are formed on the vertical CCD sections 12 through a silicon oxide film. Furthermore, a shielding film 19 composed of aluminum or tungsten is deposited through an interlayer insulating film 18, the shielding film 19 having apertures respectively formed above the photodiodes PD.
A cover film 22 is formed on the shielding film 19. The cover film 22 comprises a PSG film serving as a silicon-based passivation film, and has a refractive index of about 1.46. In addition, a planarizing layer 23, a filter layer 24, and microlenses 25 are formed on the cover film 22. The refractive index of the color filter layer 24 is about 1.5 to 1.6, and is thus substantially the same as the passivation film.
However, the solid-state imaging device (simply referred to as the “semiconductor device” hereinafter) 10′ of the second conventional example shown in FIG. 15 has the following problems:
(1) The refractive index of the cover film 22 formed above the anti-reflection film (silicon nitride film) 15 is about 1.4 to 1.6, and is greatly different from the refractive index 2.0 of the silicon nitride film serving as the anti-reflection film 15. Therefore, reflection occurs between the anti-reflection film 15 and the cover film 22.
(2) The reflection between the anti-reflection film 15 and the cover film 22 is associated with reflection from the photodiodes (light receiving elements) PD, thereby causing a smear and inhibiting an improvement in light receiving sensitivity.